Building ZeroMQ with encryption support

The code in ZeroMQ implementing the CURVE protocol utilizes crypto primitives from the NaCL library, see https://nacl.cr.yp.to/ for detailed background.

However ZeroMQ does not use NaCL itself currently. The reason for this is that the build system of NaCL assumes that the machine building the code will be the machine running the code. NaCL targets for maximal performance at the cost of portability of the binary code.

Luckily there exist two libraries providing an API compatible to NaCL(at least for the subset ZeroMQ uses):